The concept of butterfly valves with a central disk is known in the art. One such butterfly valve is shown in Steele et al. U.S. Pat. No. 4,836,499 and another is shown in Steele U.S. Pat. No. 5,295,659. In both valves a central disk pivots from a transverse or closed position, where the disk extends across a flow passage to block materials from passing through the valve, to an axial or open position, where materials enter one side of the valve and flow laterally alongside the disk face before being discharged from the opposite side of the valve. To seal the valve the disk is positioned in the transverse position and an annular elastomer seal or sealing ring is inflated to engage an edge of the disk to form an annular seal around the periphery of the disk. To pivot the disk to an open condition or flow through condition the elastomer seal is deflated and the disk is pivoted from the closed position to an open position.
One of the difficulties of handling hot material with butterfly valves is that the seals used in the butterfly valves are generally elastomers which have a low breakdown temperature. Often times the breakdown temperature of the elastomer seal is below the temperature of the material being transported. An exposure of the elastomer seal to high temperature materials, even if limited, can cause failure of the seal. Consequently, butterfly valves are used for handling materials that are at temperatures below the breakdown temperature of the elastomer seal to in order to avoid seal failure.